The present invention relates to an exhaust purifying apparatus and an exhaust purifying method for an internal combustion engine.
An exhaust purifying catalyst for an internal combustion engine that performs lean combustion such as a diesel engine, particularly, a NOx storage-reduction catalyst is poisoned by sulfur components contained in fuel. If the level of poisoning is high, the NOx storage-reduction capacity of the NOx storage-reduction catalyst is decreased. Therefore, when the NOx storage-reduction catalyst is poisoned by the sulfur components to a certain level, that is, when the sulfur components have accumulated in the NOx storage-reduction catalyst by a certain amount, a sulfur release control is performed to release the sulfur components from the catalyst. In the sulfur release control, while maintaining the catalyst bed temperature high, the air-fuel ratio of exhaust gas detected by an air-fuel ratio sensor is subjected to feedback control to be equal to either a stoichiometric air-fuel ratio or a target air-fuel ratio that is richer than the stoichiometric air-fuel ratio. Richening of the air-fuel ratio while the catalyst bed temperature is maintained high causes the sulfur components to be released from the NOx storage-reduction catalyst.
The procedure for the sulfur release control is disclosed in Japanese Laid-Open Patent Publication No. 2001-59415. Hereinafter, the sulfur release control will be described using Japanese Laid-Open Patent Publication No. 2001-59415 as an example.
According to the sulfur release control disclosed in Japanese Laid-Open Patent Publication No. 2001-59415, 700° C. conversion S release time Tre computed by the following equation (1) is used as an index for determining whether release of the sulfur components from the NOx storage-reduction catalyst performed by the sulfur release control has been completed.Tre (i)=Tre (i−1)+Ky×Tcal  (1)Where:                Tre(i): Current 700° C. conversion S release time        Tre(i−1): Previous 700° C. conversion S release time        Ky: Coefficient of sulfur release speed        Tcal: Fuel injection amount calculation cycle        
The computation of the 700° C. conversion S release time Tre using the equation (1) is performed when the air-fuel ratio of exhaust gas is equal to or richer than the stoichiometric air-fuel ratio regardless of whether the sulfur release control is being executed.
The 700° C. conversion S release time Tre computed using the equation (1) is an accumulation of time during which the air-fuel ratio of exhaust gas becomes equal to or richer than the stoichiometric air-fuel ratio and sulfur components are released, the time being converted to sulfur release time when the sulfur release control is performed with the catalyst bed temperature set to 700° C. The coefficient of sulfur release speed Ky in the equation (1) is the ratio between the release speed of the sulfur components when the catalyst bed temperature is set to 700° C. and the release speed of the sulfur components at the catalyst bed temperature of the current calculation. The coefficient of sulfur release speed Ky is obtained in accordance with the catalyst bed temperature. The fuel injection amount calculation cycle Tcal is a time interval between the previous calculation of the fuel injection amount of the internal combustion engine and the current calculation of the fuel injection amount.
After the sulfur release control is started, when the 700° C. conversion S release time Tre reaches a reference value Treo, which is a value corresponding to the time at which release of the sulfur components are completed when the catalyst bed temperature is 700° C., the sulfur release control is determined to be completed.
According to the sulfur release control disclosed in the above publication, either a slow temperature increase mode or a fast temperature increase mode is selected as the operation mode of the internal combustion engine during the control. The increasing speed of the catalyst bed temperature differs between the slow temperature increase mode and the fast temperature increase mode. More specifically, the slow temperature increase mode is selected as the operation mode immediately after the sulfur release control is started. If the 700° C. conversion S release time Tre does not reach the reference value Treo although the execution time TL of the sulfur release control in the slow temperature increase mode becomes greater than or equal to a reference value TL0, the slow temperature increase mode is switched to the fast temperature increase mode, which easily increases the catalyst bed temperature as compared to the slow temperature increase mode, to promote release of sulfur from the NOx storage-reduction catalyst.
If, for example, the air-fuel ratio sensor malfunctions and outputs only signals indicating the lean state during the feedback control of the sulfur release control, the air-fuel ratio of exhaust gas is determined to be lean although it is actually rich. Thus, addition of the 700° C. conversion S release time Tre is not performed. In this case, the 700° C. conversion S release time Tre does not reach the reference value Treo although the sulfur release control is continuously performed. Therefore, the sulfur release control cannot be ended.
In this respect, in the above publication, if the 700° C. conversion S release time Tre does not reach the reference value Treo although the actual time TL of the slow temperature increase mode has reached the reference value TL0 and the actual time TH of the subsequent fast temperature increase mode has reached the reference value TH0, the sulfur release control is determined to have caused an abnormality. As described above, by determining the existence of abnormality in the sulfur release control, measures can be taken to solve the abnormality.
However, in the above publication, the occurrence of abnormality in the control is determined only based on a fact that a predetermined time (TL0+TH0) has elapsed from when the sulfur release control has been started. The existence of abnormality is not determined in accordance with the air-fuel ratio of exhaust gas, which is directly affected by the abnormality. In other words, the existence of abnormality is determined based on a phenomenon that is indirectly caused by the abnormality, which has occurred in the sulfur release control.
In a case where the existence of an abnormality is determined based on a parameter that is indirectly affected by the abnormality that has occurred in the sulfur release control, that is, based on only the actual time of the sulfur release control, if the predetermined time (TL0+TH0) is set to a relatively short time, there may be an error in the determination of whether an abnormality has occurred in the sulfur release control. For example, the increase of the 700° C. conversion S release time Tre is delayed under circumstances where the catalyst bed temperature does not easily rise or the engine is running at a low speed during which the calculation cycle of the fuel injection amount is lengthened. In this case, although there is no abnormality in the sulfur release control, the actual time of the sulfur release control may reach the predetermined time (TL0+TH0) before the 700° C. conversion S release time Tre reaches the reference value Treo. As a result, an erroneous determination may be made that the control has caused an abnormality.
To avoid such an erroneous determination, the predetermined time (TL0+TH0) may be set longer so that the fact that the predetermined time (TL0+TH0) has elapsed from when the sulfur release control has been started reliably represents occurrence of an abnormality in the sulfur release control. However, if the predetermined time (TL0+TH0) is set longer, it takes time to make a determination as to when an abnormality actually occurs in the sulfur release control. This delays measures to be taken in response to the abnormality based on the determination result.